Preheat oven to 350°F and prepare a baking sheet by brushing with olive oil.
Prepare your onions by cutting off the ends, peel and slice thin along the curve of the onion.
Place in a bowl and toss with salt and cracked black pepper.
Heat a large pot over medium-high heat and drizzle a few glugs of olive oil in the pan followed by onions.
Toss to coat, turn heat to low and cover to slightly soften onions, about 10 minutes.
Remove lid, increase the heat and begin to caramelize onions, stirring frequently and deglazing pan periodically with white wine until it's reduced, about 20 minutes.
While onions are browning, prepare crostini.
Slice 1/4-inch thick baguette slices on a bias.
Brush with olive oil, season with salt and pepper and bake 10-12 minutes until golden.
Once onions are golden, add butter and cook until nutty.
Pour in beef stock and bring to a boil; reduce to a simmer, cooking until flavors have melded and desired consistency is achieved.
Add a splash of sherry wine (optional) for a touch of sweetness and added depth of flavor.
For soup crocks: Pour into crocks and garnish with enough crostini to cover.
Sprinkle with grated Gruyere and pop under the broiler for 3-5 minutes until golden and bubbly.
For bites: Mix together mayonnaise and mustard in a bowl and slather on baguette slices.
Spoon out 1-2 tablespoons of onions onto slathered slices, followed by a hefty sprinkling of grated cheese.
Bake in 425°F oven for 3-5 minutes until golden and bubbly.
Serve with remaining broth for dipping.